During labor and delivery, the well-being of the fetus must be carefully monitored. The procedure of monitoring the fetus allows the clinician to assess the health of the fetus, detect fetal stress, and provide appropriate treatment. Both internal (or direct) and external devices and methods are used to monitor and record such fetal parameters as heart rate, blood gas composition and pH levels during labor and delivery.
A. External Methods--Fetal Heart Rate
Several forms of external methods can be used for monitoring fetal heart rate. For example, one external method includes the use of ultrasound. An ultrasound transducer produces ultrasound waves which are directed at the fetus through the mother's abdomen. The ultrasound waves are reflected off moving fetal heart valves and the returning, reflected waves are received by the ultrasound transducer. Analysis of the returning waves and the duration between transmission and reception provide information concerning the fetal heart rate.
Another external method includes the use of a phonotransducer. This method involves placing a microphone, able to detect sound waves generated by the fetal heart, on the mother's abdomen. The phonotransducer then amplifies the sounds and translates them into an electric signal which can be monitored.
Abdominal wall electrocardiography is a third type of external fetal heart rate monitoring. This method detects fetal heart rate signals through ECG electrodes placed on the mother's abdomen. Electrical signals from both the fetal and maternal hearts are detected by the ECG electrodes. The maternal signal is filtered out and the remaining fetal heart rate signals trigger a monitor to count fetal heart beats. Proper placement of the abdominal electrodes is critical in this method; absent proper placement, electrical noise from electromyographic maternal abdominal wall muscle activity may prevent a clear signal.
All of the external methods of measuring fetal heart rate have an important advantage: they are noninvasive. Consequently, these methods largely avoid adverse effects on the mother or the fetus. The quality of the fetal heart rate recording using external monitoring methods is not as good, however, as that achieved by direct methods. This is a major limitation on external monitoring methods. As a general rule, it is necessary to restrict the mother's movements during external monitoring methods to reduce extraneous signals and interferences to obtain accurate tracings. Motion artifact is so common with external techniques that it is virtually impossible to obtain readable tracings unless logic data processing is used. Valuable information about fetal heart rate variability may be lost through such processing.
B. Direct Methods--Fetal Heart Rate
In direct fetal heart rate monitoring, an electrode is attached directly to the fetal presenting part. Typically, the electrode is a spiral wire or hook which penetrates (is inserted directly into) the fetal epidermis and holds the fetal probe in position. The primary advantage of a direct monitoring system of this kind is that the electrode detects the fetal cardiac electrical signal without interference which occurs when detecting the signal through another medium such as the mother's abdomen. The fetal cardiac electrical signal is a precise signal, allowing for accurate assessment of the fetal heart rate and any variations in that rate. Further, during direct fetal heart rate monitoring maternal movement is less restricted during the monitoring without compromising the tracing.
The limitation on this direct fetal heart rate monitoring method is that it is an invasive technique, exposing the mother and the fetus to the potential of injury, infection, or both. Injury may take the form of trauma (such as hemorrhage at the attachment site) to the skin, face, eyes, or other parts of the fetus. In addition, invasive attachment can threaten the life of the fetus by exposing the fetus to maternal body fluids containing infectious components. Venereal diseases and viruses such as acquired immune deficiency (AIDS) and hepatitis B can be transferred directly to the fetus. Moreover, the sharp wire or hook exposes the patient (mother) and clinician to potential injury.
C. Non-Invasive Direct Methods
Various techniques have been described attempting to obtain the benefits of direct fetal monitoring while avoiding the risks attendant invasive penetration of the fetal epidermis. Fetal blood gas analysis has been used to assess fetal health during labor. Blood gas analysis is typically done in a clinical laboratory on blood drawn from the fetus during labor (clearly an invasive technique). Alternatively, Okane et al., "Non-invasive continuous fetal transcutaneous pO.sub.2 and pCO.sub.2 monitoring during labor," Journal Perinatal Medicine, 17(6), 399 (1989), describe non-invasive continuous fetal transcutaneous pO.sub.2 and PCO.sub.2 monitoring during labor. Okane et al. used a commercially available device, the Micro Gas 7640 probe, available from Kontron Incorporated of Everett, Massachusetts. This probe is fixed to the fetal head using a suction ring connected to a vacuum pump which maintain a negative pressure of 200-300 mm Hg. The sensor is large and requires cervical dilation of 4 cm or more before insertion is possible. The large size of the sensor and the need to apply continuous suction, through an attached vacuum line, are deterrents to the use of the sensor.
Glue fixation of a transcutaneous pCO.sub.2 electrode for fetal monitoring has been described by S. Schmidt, "Glue fixation of the tcPco.sub.2 electrode for fetal monitoring," Journal Perinatal Medicine, 15(4), 377(1987). Glue fixation to a fetus is difficult to achieve. It requires sufficient dilation and careful preparation of the attachment site. The electrode often becomes detached during use and may need to be reapplied. In addition, trauma to the skin during removal of a sensor attached by glue is possible. Similarly, pressure-sensitive adhesives such as those used for self-adhesive bandages are hydrophobic and will not adhere to wet surfaces such as fetal skin.
Another non-invasive technique for detecting fetal ECG during labor is described by N. Randall et al., "Detection of the fetal ECG during labour by an intrauterine probe," Journal Biomedicine, (10), 159 (England 1988), and in U.S. Pat. No. 5,025,787 issued to Sutherland et al. The article and patent describe an intrauterine pressure catheter equipped with stainless steel tips which form a multi-point electrode. The intrauterine probe is inserted through the vagina into the uterine cavity. The sensor is held in contact with (but does not adhere to) the fetus by the pressure between the uterus and the fetus. The electronic signal from the sensors is processed to obtain a fetal ECG. The presence of amniotic fluid attenuates the signal from the sensors. The article points out the difficulties in obtaining accurate results due to problems with positioning the electrode tips accurately. Moreover, a degree of electrode isolation is required for optimum detection of fetal signals.
International Patent Publication Number WO 92/04864 (which claims priority of U.K. Patent Applications Number 90-20983 and Number 90-25758 by Van der Merwe) describes a fetal probe with a rigid suction cap approximately 1.5 to 2 cm in diameter. The probe incorporates a fetal heart rate sensor. A negative pressure is created in the rigid cap, by the action of a detachable piston pump, to hold the probe on the fetal skin. The pump is detached after a valve in the cap is closed. The rigid construction of the suction cap and the loss of negative pressure between the fetal skin and suction cap allow the probe to be easily detached during use.
U.S. Pat. No. 5,184,619 describes an intrauterine pressure and fetal heart rate sensor which is inserted between a fetus and the internal uterine wall following rupture of the membranes. The tubular device uses ECG electrodes as well as a pressure transducer to detect fetal heart rate and intrauterine pressure, respectively. Fetal heart rate is detected through the amniotic fluid.
To overcome the shortcomings of the external devices; the invasive, direct devices; and the non-invasive, direct devices used to measure fetal parameters during labor and delivery, a new fetal probe is provided. An object of the present invention is to attach the fetal probe securely to the fetus in a non-invasive manner. It is still another object of the present invention to assure attachment without risk of injury to the fetus, mother, or attending personnel. An additional object is to provide a probe able to transmit a clear, unattenuated signal representative of the fetal parameter being monitored.